The present invention is an improved circuit enabling pulsed biphase digital modulation of a sine wave carrier frequency, and is an improvement over the modulating circuit shown in U.S. Pat. No. 4,656,647, issued to Applicant William Hotine on Apr. 7, 1987. This patent describes a system of pulsed biphase digital modulation for a sine wave carrier and shows a diagram of the modulating circuit in its FIG. 1. Although this circuit operated satisfactorily using a high radio frequency carrier and a low frequency of digital modulation, it was found to be unsatisfactory when using the same frequency for carrier and modulation at low frequencies. At low frequencies abrupt changes in current in the inductor used to generate a lagging phase shift caused transients to be developed in the circuit. This was caused by ringing of the inductor. It was concluded-that generating phase shifts by use of pulses with an inductor was undesirable, and that a method of selecting phases from several static phase shifting circuits would not entail the use of pulses to generate the phase shifts, and would be a superior method. Fortunately there is available a CMOS integrated circuit called a quad bilateral switch, type CD4066 designed by R.C.A. This device contains four separate bilaterial switches capable of one fiftieth microsecond pulsed operation, and can switch up to 2 volts R.M.S. By using this device a novel modulating circuit has been devised, in which two of the switches are normally closed to transmit the carrier and are opened by negative pulses which are the complements of positive pulses closing the other two switches to transmit leading or lagging phase shifts during the duration of short positive pulses timed at the start and at the middle of a cycle of the carrier frequency.